Pressurised-water nuclear reactors contain a tank enclosing the nuclear reactor core and a primary circuit ensuring the circulation and cooling of the pressurised water that comes in contact with the nuclear reactor core inside the tank.
The primary circuit of the nuclear reactor includes at least one loop on which a steam generator is arranged that is linked to the tank directly by a first pipe or hot leg, ensuring the supply of pressurised water heated on contact with the fuel assemblies of the core to the primary part of the steam generator The primary part of the steam generator is also connected to the tank via a primary pump, by means of linking pipes including, in particular, a second pipe, or cold leg, of the loop of the primary circuit connected to the tank.
The pressurised cooling water of the nuclear reactor ensures, within the steam generators, the heating and vaporisation of the water supplied in order to produce vapour that drives a turbine.
The regulation of the reactivity of the nuclear reactor core, i.e., the regulation of the density of neutrons produced in the nuclear reactor core when it is operating, may be carried out, in particular, by injecting into the primary circuit, an aqueous solution containing a neutron-absorbing element such as boron 10.
For example, a solution may be used that contains boron, such as a boric acid solution that is stored in the emergency accumulators of the safety injection system or in pressurised reservoirs connected to at least one of the cold legs of the primary circuit of the nuclear reactor via an injection pipe on which means of control or regulation, such as butterfly valves, and means of injection such as one or more volumetric pumps, are arranged.
When a nuclear reactor is powered down, whether this is normal or accidental, it is necessary to evacuate the residual power from the core in order to avoid the melting of the fuel assemblies.
Additionally, a borication of the powered-down primary reactor circuit is necessary to control the reactivity of the core.
These functions must be ensured even if the electrical power supplies of the nuclear plant are unavailable.
In the event of total loss of electrical power supplies and cooling systems, it is necessary to start up, as soon as possible, a means of continuous injection of water containing a neutron-absorbing element, e.g., one containing a minimum of 2500 ppm of boron that has a natural isotope content of boron 10, and a flow rate of between 20 and 90 m3/hour. This injection of water containing a neutron-absorbing element, and in particular boron 10, must occur very quickly after the reactor is powered down; if it is delayed, the meltdown of the fuel assemblies will have begun.
If the degradation of the fuel assemblies cannot be avoided, the injection of water containing a neutron-absorbing element remains necessary in order to maintain the core in a subcritical, cooled state. The necessary flow rate changes with the residual power present in the nuclear reactor core.
Accident management scenarios foreseen to date take into account the total loss of electrical power supplies in existing cooling systems, or the total loss of the heat sink, due, e.g., to a breach of the main primary circuit.
However, these scenarios do not take into account the combination of the two.
In order to remedy such a situation, it is necessary to evacuate the residual power from the core in order to avoid the meltdown of the fuel assemblies by means of continuous injection of water containing a neutron-absorbing element consisting of boron.
One solution is to use the tarpaulins present on the site and fill them continuously, in this emergency situation, with a mixture of water and boron.
However, the difficulty is the transportation of a substantial amount of water containing boron, prepared outside of the area of the nuclear reactor, in a crisis situation.